1. Field of the Invention
This invention relates to the atmospheric venting of vapors from cryogens, and more particularly, to the venting of cold, relatively dense natural gas emanating from a confined body of liquefied natural gas.
2. Brief Description of the Prior Art
The hazard of venting heavy gases or vapors from storage facilities or other locations containing stored liquids or vapors is frequently encountered in industry. A particular situation often experienced is the venting from storage tanks of explosive fumes or vapors derived from volatile liquids contained within the storage tank, with such venting occurring at such times as a sufficient pressure builds within the storage tank to actuate a safety relief valve or other device of similar character. Hydrocarbon vapors which are flammable in admixture with air are frequently sufficiently dense that they accumulate along or over the ground adjacent storage tanks, or other points of venting, and constitute a safety hazard.
It has been proposed in U.S. Pat. No. 2,866,475 to Sutherlin et al. to dilute volatile, flammable vapors by admixing atmospheric air with such vapors as they are vented from a stack or other discharge conduit disposed at the top of a storage tank for volatile liquids. A venting system is provided in which the flow of the flammable hydrocarbon vapors through a nozzle in the stack serves to induce atmospheric air to flow through a Venturi inlet passageway and become admixed with the hydrocarbon vapors. This results in a dilution of the vapors as they are discharged from the stack into the surrounding atmosphere, so that the concentration of vapors in air is reduced to below ignition concentrations and the danger of explosion is reduced or obviated. This method of dilution of some types of flammable hydrocarbon vapors with air is undesirable, however, because in reaching a low level of concentration where flammability and explosion are no longer likely, the mixture must pass through an explosive range of concentration of the vapors in air, thereby constituting at least a temporary explosion hazard. The hazard arising from the transition of the mixture through the explosive range is enhanced by the fact that often, rapid venting through a relatively small stack conduit occurs, and static charges building up on the top of the tank or such conduit can easily ignite the explosive mixture thus formed.
One specific instance where venting of flammable vapors has previously constituted a safety hazard is that involving the venting of cold natural gas from storage tanks containing liquefied natural gas, hereinafter referred to as LNG. Such storage tanks are normally provided with pressure relief valves which open to allow escape of vapors when the tank contents become unstable due to rollover, stratification or other causes. The pressure relief valves employed will open under a pressure differential of from about 1 psi to about 2 psi between the interior of the tank and the ambient environment. This sensitivity will allow opening of such valves even upon significant changes in barometric pressure.
In LNG storage tanks, the natural gas in equilibrium with the LNG is cold and is more dense than air. This gas, when vented, will therefore move downwardly to ground level, or at least will be slow to dissipate by upward movement into the atmosphere. An attempt to dilute the natural gas vapors to below ignition concentrations with air in the manner described in U.S. Pat. No. 2,866,475 is hazardous because in accomplishing dilution to this level, the mixture must pass through the explosive range (5 - 15 percent of the natural gas in air). The danger is amplified by the real possibility of sparking originating with the build-up of static charges on the tank and vent stack. An explosion at or within the pressure relief valve can shear this structure from the tank, thereby exposing the entire tank contents to possible ignition. Other stored cryogenic materials can pose similar problems attributable to the density of the cold vapors, and their flammability, or in some instances, toxicity.
A similarly hazardous release of natural gas vapors has occurred in natural gas liquefaction plants when the plant is brought off-stream. At this time the content of the "cold box" is vented through the stack, and fires frequently occur at this location. Such fires have been controlled by nitrogen snuffers which dilute the vapors below the flammability limit by the use of large amounts of nitrogen gas.